Silver halide photographic elements contain light sensitive silver halide in a hydrophilic emulsion. An image is formed in the element by exposing the silver halide to light, or to other actinic radiation, and developing the exposed silver halide to reduce it to elemental silver.
In color photographic elements, a dye image is formed as a consequence of silver halide development by one of several different processes. The most common is to allow a by-product of silver halide development, oxidized silver halide developing agent, to react with a dye forming compound called a coupler. The silver and unreacted silver halide are then removed from the photographic element, leaving a dye image.
In either case, formation of the image commonly involves liquid processing with aqueous solutions that must penetrate the surface of the element to come into contact with silver halide and coupler. Thus, gelatin, and similar natural or synthetic hydrophilic polymers, have proven to be the binders of choice for silver halide photographic elements. Unfortunately, when gelatin, and similar polymers, are formulated so as to facilitate contact between the silver halide crystal and aqueous processing solutions, they are not as tough and mar-resistant as would be desired for something that is handled in the way that an imaged photographic element may be handled. Thus, fingerprints can easily mark the imaged element, it can be scratched or torn and it can swell or otherwise deform when it is contacted with liquids.
There have been attempts over the years to provide protective layers for gelatin based photographic systems that will protect the images from damages by water or aqueous solutions. U.S. Pat. No. 2,173,480 describes a method of applying a colloidal suspension to moist film as the last step of photographic processing before drying. A series of patents describes methods of solvent coating a protective layer on the image after photographic processing is completed and are described in U.S. Pat. Nos. 2,259,009, 2,331,746, 2,798,004, 3,113,867, 3,190,197, 3,415,670 and 3,733,293. U.S. Pat. No.5,376,434 describes a protective layer formed on a photographic print by coating and drying a latex on a gelatin-containing layer bearing an image. The latex is a resin having a glass transition temperature of from 30.degree. C. to 70.degree. C. The application of UV-polymerizable monomers and oligomers on processed image followed by radiation exposure to form crosslinked protective layer is described in U.S. Pat. Nos. 4,092,173, 4,171,979, 4,333,998 and 4,426,431. One drawback for the solvent coating method and the radiation cure method is the health and environmental concern of those chemicals to the coating operator. The other drawback is that these materials need to be coated after the processing step. Thus, the processing equipment needs to be modified as well as the personnel running the processing operation need to be trained. In addition, several lamination techniques are known and practiced in the trade.
U.S. Pat. Nos. 3,397,980, 3,697,277 and 4,999,266 describe methods of laminating polymeric sheet film on the processed image as the protective layer. U.S. Pat. No. 5,447,832 describes the use of a protective layer containing a mixture of high and low Tg latices as the water-resistant layer to preserve the antistat property of the V.sub.2 O.sub.5 layer through photographic processing. This protective layer is not applicable to the image formation layers since it will detrimentally inhibit the photographic processing. U.S. Pat. No. 3,443,946 provides a roughened (matte) scratch-protective layer, but not a water-impermeable one. U.S. Pat. No. 3,502,501 provides protection against mechanical damage only; the layer in question contains a majority of hydrophilic polymeric materials, and must be permeable to water in order to maintain processability. U.S. Pat. No. 5,179,147 likewise provides a layer that is not water-protective. However, all these techniques need to be carried out after the image has been formed, which adds a large cost to the final imaged product.
Protective coatings that need to be applied to the image after it is formed, several of which were mentioned above, adds a significant cost to the final imaged product. A number of patents have been directed to water-resistant protective coatings that can be applied to a photographic element prior to development. For example, U.S. Pat. No. 2,706,686 describes the formation of a lacquer finish for photographic emulsions, with the aim of providing water- and fingerprint-resistance by coating the light-sensitive layer, prior to exposure, with a porous layer that has a high degree of water permeability to the processing solutions. After processing, the lacquer layer is fused and coalesced into a continuous, impervious coating. The porous layer is achieved by coating a mixture of a lacquer and a solid removable extender (ammonium carbonate), and removing the extender by sublimation or dissolution during processing. The overcoat as described is coated as a suspension in an organic solvent, and thus is not desirable for large-scale application. More recently, U.S. Pat. No. 5,853,926 to Bohan et al. discloses a protective coating for a photographic element, involving the application of an aqueous coating comprising polymer particles and a soft polymer latex binder. This coating allows for appropriate diffusion of photographic processing solutions, and does not require a coating operation after exposure and processing. Again, however, the hydrophobic polymer particles must be fused to form a protective coating that is continuous and water-impermeable.
U.S. Pat. No. 5,856,051 describes the use of hydrophobic particles with gelatin as the binder in an overcoat formulation. This invention demonstrated an aqueous coatable, water-resistant protective overcoat that can be incorporated into the photographic product, allows for appropriate diffusion of photographic processing solutions, and does not require a coating operation after exposure and processing. The hydrophobic polymers exemplified in U.S. Pat. No. 5,856,051 include polyethylene having a melting temperature (Tm) of 55 to 200.degree. C., and therefore capable of forming a water-resistant layer by fusing the layer at a temperature higher than the Tm of the polymer after the sample has been processed to generate the image. The coating solution is aqueous and can be incorporated in the manufacturing coating operation without any equipment modification. The fusing step is simple and environmentally friendly to photofinishing laboratories. Since the particles are incorporated entirely within the uppermost layer, this approach does not suffer from a lack of mechanical strength and integrity during transport and handling prior to image formation and fusing. However, the scratch resistance of such an overcoat after fusing is a serious concern, since polyethylene is a very soft material.
Similarly, commonly assigned U.S. Ser. No. 09/353,939 (Docket 79581) and U.S. Ser. No. 09/548,514 (docket 80493), respectively, describe the use of a polystyrene-based material and a polyurethane-based material, with gelatin as the binder, in an overcoat for a photographic element, which overcoat can be fused into a water resistant overcoat after photographic processing is accomplished to generate an image. Finally, U.S. Ser. No. 09/548,514 describe the use of epoxy materials in the overcoat with gelatin as the binder, which similar to polyethylene, polystyrene and polyurethane can be fused into a water resistant overcoat after photographic processing is completed.
Commonly assigned U.S. Ser. No. 09/235,436 discloses the use of a processing solution permeable overcoat that is composed of a urethane-vinyl copolymer having acid functionalities. Commonly assigned U.S. Ser. No. 09/235,437 and U.S. Ser. No. 09/448,213 (Docket 80220) disclose the use of a second polymer such as a gelatin or polyvinyl alcohol to reduce such defects and disadvantages.
A problem with these photographic elements, which have overcoats provided at the point of manufacture (and hence must be permeable to aqueous solutions during the processing step) is that the post-process photographic element with a water/stain resistant overcoat may exhibit unsatisfactory abrasion resistance in the wet state. In particular, if allowed to come into contact with aqueous solutions for a sufficient length of time such that the underlying hydrophilic layers can swell, the photographic element becomes prone to abrasion when wiped with a dry cloth or paper towel. Therefore, there is a need to improve the wet abrasion resistance of the imaged element bearing the aforementioned water resistant protective overcoat.